Implements supported on machines, and the machines carrying the implements, should normally be operated to achieve maximum productivity. Earthmoving machines, and implements on these machines, are prime examples of such devices. The productivity or production rate for these machines can be defined as the volume of soil moved per unit time multiplied by the distance over which the soil is moved for a given working or soil condition environment. This, and other definitions of productivity, are known and used in the art. In machines and implements that are manipulated by a human operator, the skill of the operator is a practical limitation to attaining maximum productivity. Productivity usually is lower with unskilled operators than with skilled operators. For example, an unskilled operator may achieve as little as 65% of the productivity obtained by a highly skilled operator using the same machine.
Maximum productivity can be achieved by maximizing the "draft power" of the earthmoving machine. Draft power is the rate of actual useful work being done in moving the soil and is defined as the product of the draft force of the earthmoving implement and the ground speed of the earthmoving machine. A track/wheel bulldozer and a bulldozer blade constitute one type of earthmoving machine and implement that moves or pushes soil. For these devices, draft force is the force on the blade and ground speed is the bulldozer ground speed.
A simple example of a working condition is the operation of the bulldozer to level an area. As the bulldozer starts forward with the blade elevated, draft power is zero since draft force is zero. As the blade is lowered and cuts into the soil, draft force increases and, hence, draft power increases. As the blade cuts deeper, draft force may continue to rise, but ground speed may decrease. Maximum draft power is reached when the bulldozer is moving at maximum ground speed commensurate with draft force.
Control systems have been developed that provide information for controlling the blade during various working conditions. These include control systems disclosed in (1) U.S. Pat. Nos. 4,194,574 by Benson et al., issued Mar. 25, 1980; (2) 4,166,506 by Tezuka et al., issued Sept. 4, 1979; and, (3) 4,157,118 by Suganami et al., issued June 5, 1979. A common problem with these control systems is the inability to adequately maintain stable blade control over the entire working area of the bulldozer. While stable blade control may be maintained when the bulldozer and blade are being operated over a substantially level or horizontal area, the problem arises when the bulldozer pitches forward into a cut and then pitches aft on ascending the other side of the cut. Upon pitching forward into the cut, the blade can quickly cut more deeply into the soil and become overloaded, and upon pitching aft the blade can move totally out of the soil and become unloaded or leave underneath a substantial amount of soil that had been carried during the cut. At the time of pitching, either forward or aft, the earthmoving machine has a substantial longitudinal angular velocity.
Whereas the information provided by the prior control systems may be useful for controlling the blade during the level portion of the cut, this information is not satisfactory for controlling the blade during the pitching conditions. For example, in U.S. Pat. No. 4,194,574, the information is an audible or visual representation of the blade power. The operator must respond to this data by manually moving a control lever to hydraulically raise the blade upon the forward pitching to compensate for the downward blade movement or to lower the blade upon aft pitching to compensate for the upward blade movement. Not only is the operator response to this information slow when a quicker response time is needed during the pitching conditions, but the operator can overshoot or undershoot the proper blade position, causing blade oscillation. Moreover, productivity is reduced during these pitching conditions because maximum blade power is not achieved.
Other disadvantages occur with the prior blade control systems, whether the bulldozer and blade are being controlled over a level area or during the pitching conditions. In U.S. Pat. No. 4,194,574, the control system senses blade force and bulldozer ground speed, and then calculates blade power. This information controls, for example, a variable rate audible signal generator whose audible tone rate varies as the calculated power changes. The operator must then manually move a control lever that controls a hydraulic actuator which, in turn, controls a lift cylinder that moves the blade. This manual control is performed in an attempt to achieve maximum blade power, which is indicated when a predetermined tone is produced by the signal generator.
One problem with the system of the '574 patent is the relatively quick onset of operator fatigue, both mental and physical, in responding to the alarm signal generator and moving the control lever to control the hydraulic actuator. For example, a percentage of operator lever control movement does not result in lift cylinder movement to reposition the blade. This is because the operator has not moved the control lever far enough to overcome cylinder pressure due to blade load. Also, a percentage of the control lever movements overshoot or undershoot the lever position corresponding to maximum blade power. Furthermore, the undercarriage life of the bulldozer is reduced owing to the occurrence of excessive and repeated track/wheel slippage, resulting in reduced ground speed, until the operator can manipulate the lever to again achieve maximum blade power.
In U.S. Pat. No. 4,166,506, the control system is designed to maintain a constant, predetermined load or force on the blade and not to control blade power. This is not sufficient to optimize productivity. This system senses the actual variable load, compare the sensed load to a predetermined fixed load, and produces control information to automatically raise or lower the blade in response to the comparison until the actual and predetermined loads are equal. The use of the predetermined fixed load also has the disadvantage of not allowing the operator to vary the setting of this important parameter which is directly related to blade power. The option to select a parameter directly related to blade power is beneficial when dictated by changing soil conditions and terrain irregularities. For example, for harder soil, it is beneficial to operate the blade under higher loads than the predetermined load.
The U.S. Pat. No. 4,157,118 has a control system in which the operator selects a desired or command blade height relative to the soil or depth of cut, which is then compared to the actual blade height according to sensed height data. The blade is then raised or lowered automatically until the command blade height and actual blade height are the same. Actual blade load is not sensed directly, but is calculated in response to engine speed and throttle opening and compared with a maximum preset load which is dictated by the particular working conditions. Should the load of the blade exceed the preset maximum load when the blade is at the commanded height, the control system overrides the height control and automatically causes the blade to rise until the actual load falls below the maximum load. As with the '506 patent, the control system of the '118 patent is not designed to control blade power, but rather blade height and maximum blade force or load. The latter, for example, may be preset too low if blade power were taken into consideration. Furthermore, the blade load control feature can function only to raise the blade and not to lower the blade.
The present invention is directed to overcoming one or more of the problems as set forth above.